mfd: omap-usb-tll: Fix register offsets

gcc-8 notices that the register number calculation is wrong
when the offset is an 'u8' but the number is larger than 256:

drivers/mfd/omap-usb-tll.c: In function 'omap_tll_init':
drivers/mfd/omap-usb-tll.c:90:46: error: overflow in conversion from 'int' to 'u8 {aka unsigned char}' chages value from 'i * 256 + 2070' to '22' [-Werror=overflow]

This addresses it by always using a 32-bit offset number for
the register. This is apparently an old problem that previous
compilers did not find.

Fixes: 16fa3dc7 ("mfd: omap-usb-tll: HOST TLL platform driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>